🧪 Implement tests for _step_sources function

[edithatogo]

🎯 What: The _step_sources function in pic_contracts/traces.py was previously untested. This function extracts the trace path mapped to source references for each step.
📊 Coverage: A new test test_step_sources has been added covering:

• Happy path (valid stepId and sourceRefs)
• Empty trace edge case
• Trace missing steps key
• Steps missing stepId or sourceRefs keys
  ✨ Result: Enhanced unit test coverage ensuring that source reference mappings are reliably parsed.

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PR created automatically by Jules for task 12802438962781962567 started by @edithatogo

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[amazon-q-developer]

This PR adds comprehensive test coverage for the _step_sources function with well-structured test cases covering the happy path and multiple edge cases. The tests correctly validate the function's behavior with missing keys and malformed data. The implementation is solid with no blocking issues identified.

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